Steel blogger

It’s simple to assume that “GI pipe” and “MS pipe” refer to entirely different metals. Both pipes use steel as the base material, and the surface treatment creates the primary distinction. However, this small difference has significant implications for durability, mechanical performance, and project selection. This article compares the mechanical characteristics of GI (galvanized iron) and MS (mild steel) pipes and explains what they mean for engineers, contractors, and purchasers.

What we mean by GI and MS pipes

• MS pipes are made from mild (low-carbon) steel. They are typically produced as welded (ERW) or seamless pipes and supplied with an untreated steel surface.
• GI pipes are MS pipes that have been coated with zinc (usually by hot-dip galvanizing or electro-galvanizing) to protect the steel from corrosion. The zinc layer sits on top of the same steel substrate that’s used for MS pipes.

Because the core metal is often similar, many base mechanical properties are comparable — but the coating, manufacture route and intended use introduce meaningful differences.

Tensile strength and yield strength

Rather than the zinc coating, the steel grade used to make GI and MS pipes largely determines their tensile and yield strengths. The yield strengths of typical low-carbon mild steels used for pipes are approximately 250 MPa, while the tensile strengths are typically in the mid-hundreds of MPa (exact values vary by grade and supplier). In actuality:

• MS pipe (bare) — mechanical strengths match the base steel and will be consistent with the pipe standard or manufacturer spec.
• GI pipe — the zinc coating itself contributes negligibly to tensile or yield strength. Hot-dip galvanizing is performed after fabrication and does not significantly change core mechanical properties for mild steel. Therefore, for the same steel grade and wall thickness, GI and MS pipes will behave similarly under static tension.

Ductility, elongation and toughness

Ductility (ability to deform plastically before fracture) and toughness are functions of base steel chemistry and microstructure.

• If both pipes use the same mild steel grade, ductility and toughness are comparable.
• Note: aggressive heating or local surface defects from poor galvanizing practice can introduce stress concentrators; good quality galvanizing avoids this. For impact-sensitive applications, always check supplier test certificates.

Hardness and wear resistance

The zinc layer on GI pipes is softer than the steel substrate. That means:

• Surface hardness of GI pipes is lower at the coating level, but the structural steel hardness remains unchanged.
• For applications where surface abrasion or wear is a concern (e.g., exposed sliding contact), both MS and GI pipes behave like mild steel; the zinc does not improve wear resistance and can actually wear off.

Corrosion resistance — the practical mechanical effect

Corrosion isn’t a “mechanical property” in the strict sense, but it directly affects structural performance over time.

• MS pipes corrode when exposed to moisture/chemicals; corrosion reduces wall thickness, which lowers load capacity, burst pressure and fatigue life.
• GI pipes have a sacrificial zinc layer that prevents or slows corrosion—this preserves the pipe’s effective wall thickness far longer. Mechanically, that means GI pipes will maintain strength, stiffness and safety margins for a longer service life in corrosive environments.

Fatigue resistance and long-term performance

Fatigue life depends strongly on surface condition and the presence of corrosion pits.

• In corrosive or cyclic loading conditions, GI pipes usually outperform bare MS pipes because the zinc delays pitting and maintains smoother surfaces, reducing crack initiation sites.
• Under purely mechanical cyclic loads in non-corrosive conditions, fatigue performance is similar if the base steel and surface finish are equivalent.

Weldability and formability

Mild steel is easy to weld and bend; galvanizing adds a small wrinkle.

• MS pipes (bare): excellent weldability and formability.
• You can weld and form GI pipes, but welding through zinc produces hazardous zinc fumes and requires you to remove the zinc at the weld zone or use proper fume extraction and PPE. Bending and threading are possible but the zinc coating may crack or flake at the bend, requiring touch-up coating.

Choosing between GI and MS pipes — practical guidance

• Choose MS pipe when corrosion is not a concern (indoor, dry, buried with protective measures) and when you want cheaper upfront cost or easier welding without coating removal.
• Choose GI pipe when exposure to moisture, outdoor use, plumbing, irrigation or buried applications make corrosion protection important. The preserved wall thickness and improved long-term mechanical performance usually justify the premium.

Final takeaway

The base material controls tensile strength, yield, and ductility because GI and MS pipes share the same steel backbone. By preventing corrosion, extending fatigue life in corrosive environments, and maintaining structural capacity, the zinc coating on GI pipes significantly improves the pipe’s long-term mechanical reliability without significantly altering those core numbers. Choose according to the environment, necessary service life, and welding/installation requirements; before making a purchase, always verify supplier certifications for steel grade and galvanizing quality.